This invention relates to a method of manufacturing a cylinder which can be adapted to a front fork providing an internal damping device on a motorcycle for example.
JP-A5-65930 is known as an example of a front fork for a motorcycle.
This front fork comprises an outer tube in which an inner tube is inserted in to slide freely and a damper is arranged to rise from the bottom of the inner tube. Thick sections are formed on both ends of the cylinder comprising the damper by the threadable engagement of the cylinder with a cylinder head and a valve body. A cutting operation is applied to reduce the thickness of the middle section between the thick sections of the cylinder in order to decrease the overall component weight.
However since a reinforcing material is used in order to maintain the required cylinder strength, the problem arises that the cost of the reinforcing material is high due to its high strength. Furthermore since a section of this expensive material is cut away, material is wasted and it is difficult to reduce costs. In addition, productivity is adversely affected due to the fact that the number of processing steps is increased and problems arise with respect to the adhesion between the materials.
It is therefore an object of this invention to provide a method of manufacturing a cylinder which displays improved processing characteristics, improved adhesion and avoids wastage of materials.
The method according to this invention is a method of manufacturing a cylinder comprising, a step of inserting a cored bar into piping material having a reduced inner radius at one end, a step of supporting the reduced radius section of the piping material with the inserted tip of the cored bar, and a step of spinning the piping material in which rotating the piping material and the cored bar, displacing passively rotating one or a plurality of rollers in an axial direction while compressing the outer periphery of the piping material, and pressuring and retaining the piping material between the cored bar and the roller; wherein the thickness of the middle section of the piping material is made less than the thickness of the end sections by moving the roller into a position closer to the cored bar in the middle section than in the end sections of the piping material.